A standard prefilled hypodermic syringe has a tubular body whose front end is fitted with a needle or cannula, whose rear end is blocked by a slidable plunger, and which is fitted over the needle with a protective cap. Normally such a syringe is filled from the back, that is the assembled body, needle, and cap are held in an upright position and an automated device fills the required medicament into the body, whereupon the plunger is installed.
The subassembly comprised of the body, needle, and end cap is typically made up in a high-speed machine. The needle is fitted to the front end of the body, then the cap is fitted over the needle, all as mentioned by automatic equipment at relatively high speed.
A common problem is that the needle is set so that it is not perfectly aligned with the centerline of the body. Thus when the cap is installed the needle can touch, poke into, or even poke through the cap. This clearly creates a potentially unsanitary condition in that bacteria can enter the otherwise sealed assembly through the hole in the cap created by the misaligned needle.
The standard procedure is simply to have a worker visually examine the needle assemblies before final packaging and to pick out and discard any bad subassemblies. Such a system is, clearly, labor intensive and cannot be guaranteed to catch every bad needle subassembly, especially when the needle does not project all the way through the wall of the cap or, after poking through it, has somehow withdrawn inside to leave a hole that is nearly impossible to see. Finally it is in fact desirable to reject any subassemblies where the needle is simply too close to the cap, within say 0.1 mm to 0.2 mm, as during subsequent transport and handling the needle might well come into contact with the cap wall and pierce it.